Connexin-based gap junction hemichannels: gating mechanisms

Abstract

Connexins (Cxs) form hemichannels and gap junction channels. Each gap junction channel is composed of two hemichannels, also termed connexons, one from each of the coupled cells. Hemichannels are hexamers assembled in the ER, the Golgi, or a post Golgi compartment. They are transported to the cell surface in vesicles and inserted by vesicle fusion, and then dock with a hemichannel in an apposed membrane to form a cell-cell channel. It was thought that hemichannels should remain closed until docking with another hemichannel because of the leak they would provide if their permeability and conductance were like those of their corresponding cell-cell channels. Now it is clear that hemichannels formed by a number of different connexins can open in at least some cells with a finite if low probability, and that their opening can be modulated under various physiological and pathological conditions. Hemichannels open in different kinds of cells in culture with conductance and permeability properties predictable from those of the corresponding gap junction channels. Cx43 hemichannels are preferentially closed in cultured cells under resting conditions, but their open probability can be increased by the application of positive voltages and by changes in protein phosphorylation and/or redox state. In addition, increased activity can result from the recruitment of hemichannels to the plasma membrane as seen in metabolically inhibited astrocytes. Mutations of connexins that increase hemichannel open probability may explain cellular degeneration in several hereditary diseases. Taken together, the data indicate that hemichannels are gated by multiple mechanisms that independently or cooperatively affect their open probability under physiological as well as pathological conditions.

Fig. 1

Diagram of hemichannels depicting two putative covalent modifications, disulfide bond formation and phosphorylation, that the regulate open probability ( P_o) of Cx43 hemichannels. Under normal conditions, most cysteines are in their reduced form, most potential sites are phosphorylated, and hemichannel P_o is low. Dephosphorylation under normal conditions increases P_o (left hemichannel). Metabolic inhibition generates reactive oxygen species, e.g., nitric oxide, that oxidize cystines to cysteines and increase P_o. Dephosphorylation induced by metabolic inhibition appears not to be necessary for this opening (right hemichannel). Under normal conditions, reducing agents, DTT and GSH, increase P_o, perhaps by reducing disulfide bonds formed under basal conditions (lowest hemichannel). Phosphorylation state may account for different effects of reducing agents in normal conditions and under metabolic inhibition. Reducing agents may produce different covalent changes depending on condition. Modulator proteins may participate in these actions. P: phosphate group on Cx43. –S–S–: disulfide bond. ROS: reactive oxygen species.